Evolutionary Analysis of the Highly Conserved Insect Odorant Coreceptor (Orco) Revealed a Positive Selection Mode, Implying Functional Flexibility

Abstract

Odorant coreceptor (Orco) represents one of the essential genes in the insect olfactory system, which facilitates signal transduction and heterodimerization with different odorant receptors (Ors) in the insect antennal dendritic membrane. Evolutionary analysis by detecting positive selection is important to examine the functional flexibility of Orco that potentially supports insect survival. The maximum likelihood codon substitution model was applied using CODEML program as implemented in PAML ver 4.9e package across 59 Orco codon sequences available from GenBank. These sequences represented five major insect orders and two reproductive systems (holometabola and nonholometabola). In the site model that identified common ω values for Orco, it was clearly shown that Orco was under strong purifying selection, indicated by the ω value that was far from 1 (ω: 0.03). However, in to the branch model, positive selection was detected to be acting on Dipteran lineages, whereas in the branch-site model, several sites were under significant positive selection occurring in the following four clades: Coleoptera, Diptera, Lepidoptera, and Psocodea. The typical evolutionary mode acting on Orco was consistent with the entropy value [H(x)], confirming that 48.9% of the Orco site was under conservation (H(x) < 0.5), whereas 26.9% of the Orco sites was under high variation (H(x) > 1). These findings confirmed that Orco genes are generally highly conserved and can possibly be used for the manipulation of insect pest control programs. However, positive selection that acts on certain lineages suggested future adaptive evolutionary ability of Orco to anticipate flexible functions for successful olfactory processes.

Functional properties of insect olfactory receptors: ionotropic receptors and odorant receptors

“…Furthermore, in winged insects there was a massive expansion of OR genes (Missbach et al 2014 ; Thoma et al 2019 ). These ORs are heteromeric constructs containing variable, odor-specific OrX proteins, and highly conserved co-receptor proteins (Orco) (Larsson, et al 2004 ; Neuhaus et al 2005 ; Soffan et al 2018 ). Compared with the more ancient IRs, the ORs are more sensitive and are capable of resolving fast changing odor pulses (Getahun et al 2012 ).…”

“…The OrX show a high degree of variability which usually reflects the ecological niche of a given species. By contrast, Orco proteins are quite conserved among species and they form a separate clade within OR proteins (Soffan et al 2018 ). ORs represent ligand-gated ion channels (Sato et al 2008 ; Wicher et al 2008 ).…”

Insect Odorscapes: From Plant Volatiles to Natural Olfactory Scenes

“…Typically, ORs have a seven-transmembrane topology and form heterodimers with a co-receptor named Orco (Butterwick et al, 2018). While Orco is fully conserved across species, sharing up to 94% sequence identity with orthologs from different species (Butterwick et al, 2018; Soffan et al, 2018), the sequence identity of the other ORs can vary greatly within and between species (Hansson and Stensmyr, 2011). Additionally, the size of the OR repertoire, i.e., the content of OR sequences in a genome, differs from one species to another, reflecting their ecology and lifestyle as well as the evolutionary history of the species.…”

Whole-exome sequencing identifies novel mutations in ABC transporter genes associated with intrahepatic cholestasis of pregnancy disease: a case-control study

“…We also selected the same twelve loci as Sanger sequencing for evolutionary conservation analysis for the same reasons. Evolutionary conservation analysis [32,33] of the amino acids encoded by the new pathogenicity sites in ABC genes was performed among vertebrates, including Gibbon, Macaque, Olive Baboon, Gelada, Marmoset, Mouse, Rat, Cow, Sheep, Pig, Chicken, Zebra Finch, and Zebrafish, using genomic alignments of the Ensembl Genome Browser.…”